My layman’s understanding is that your immune system has an “strong opinion” about what bacteria it will tolerate, so simply introducing new bacteria generally

My layman’s understanding is that your immune system has an “strong opinion” about what bacteria it will tolerate, so simply introducing new bacteria generally does not make a permanent change to your gut biome. Even fecal transplant doesn’t really work for transforming the gut biome, as I understand it. (It can help reestablish bacteria that were lost due to medication or disease though.)